On Christmas Eve, a Department of Foreign Affairs and Trade spokesman confirmed Ms Armstrong had boarded a flight out of Mongolia.

She landed at Sydney Airport yesterday afternoon and was due to get a connecting flight to Tasmania to be reunited with her parents.

SouthGobi Resources on Monday said Mongolia's Independent Authority Against Corruption (IAAC) had ended its questioning of the lawyer.

The IAAC informed SouthGobi that the 32-year-old "is no longer a suspect in their investigations", the coal firm said in a statement to the Hong Kong stock exchange where it is listed.

It is understood that Foreign Minister Bob Carr on Friday contacted the Mongolian ambassador to Australia about Ms Armstrong's case.

Senator Carr said in a statement on Monday that Ms Armstrong being allowed to leave was "great news" and good timing so she could see her family for Christmas.

Mongolian officials said Ms Armstrong was questioned over an investigation into the former chief of Mongolia's mining authority, who is suspected of illegally handling mining licences, according to Dow Jones Newswires.

Her mother Yvonne Armstrong told AAP on Monday that her daughter's release was a Christmas miracle and the detention had taken its toll on her daughter, who had been travelling to Mongolia for years.

An attache at the Mongolian Embassy in Canberra, Hantulga Galaazagraa, told the ABC the investigation had been finalised and it was nothing against Ms Armstrong personally.

He said she had been asked not to leave the country because she might have useful information about the investigation for the anti-corruption agency.

It was believed SouthGobi Resources had a tax avoidance issue, Mr Galaazagraa said.